Context. Major mergers of gas-rich galaxies provide promising conditions for the formation of supermassive black holes (SMBHs; ≳105 M☉) by direct collapse because they can trigger mass inflows as high as 104 - 105 M☉ yr-1 on sub-parsec scales. However, the channel of SMBH formation in this case, either dark collapse (direct collapse without prior stellar phase) or supermassive star (SMS; ≳104 M☉), remains unknown.
Aims: Here, we investigate the limit in accretion rate up to which stars can maintain hydrostatic equilibrium.
Methods: We compute hydrostatic models of SMSs accreting at 1-1000 M☉ yr-1, and estimate the departures from equilibrium a posteriori by taking into account the finite speed of sound.
Results: We find that stars accreting above the atomic cooling limit (≳10 M☉ yr-1) can only maintain hydrostatic equilibrium once they are supermassive. In this case, they evolve adiabatically with a hylotropic structure, that is, entropy is locally conserved and scales with the square root of the mass coordinate.
Conclusions: Our results imply that stars can only become supermassive by accretion at the rates of atomically cooled haloes (̃0.1 - 10 M☉ yr-1). Once they are supermassive, larger rates are possible.
Astronomy and Astrophysics
- Pub Date:
- December 2019
- stars: massive;
- early Universe;
- quasars: supermassive black holes;
- accretion disks;
- Astrophysics - Solar and Stellar Astrophysics;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - High Energy Astrophysical Phenomena
- 5 pages, 3 figures